So, you don't seem to know the difference between sysadmins (people that keep servers running) and developers (people who write the software that run on servers

So, you don't seem to know the difference between sysadmins (people that keep servers running) and developers (people who write the software that run on servers). This is forgivable, since we kinda conflate the skillsets these days (lol devops). Anyways, there are kinda two situations: 1. Engineer getting bored and leaving. 2. Engineer getting hit by bus. The first case can be mitigated by giving significant equity and by including them in important business decisions, and also by letting them try new things (within reason, away from the core of the business). The second case can be mitigated by encouraging them to document everything really well and to teach you how the system works, and also by giving them a junior developer or intern to train. Additionally, have procedures in place for continuity of passwords, credentials, etc. in case of disaster. Honestly, you're going to have enough trouble getting the business off the ground that being worried about this particular thing isn't really warranted until you know the market cares about you. Good luck!